Numerous types of hammermills and other kinds of machines for grinding, disintegrating or shredding are known for the pulverizing, shredding, defiberizing or flaking of various types of materials. There are numerous applications which a single rotor hammermill is not capable of performing satisfactorily, so that a plurality of single rotor hammermills are used in succession. One problem encountered generally in single rotor hammermills, utilized for treating waste material, is their inability to pulverize or comminute waste paper when in wet condition, as well as the tremendous cost of maintenance, often requiring the installation of approximately 50% greater capacity, because of the necessity for frequently rewelding the hammers, such as after only 16 hours of operation. In an attempt to overcome at least some of the difficulties of single rotor hammermills, double rotor hammermills, in a horizontal arrangement, have been designed but have proven to be a good compromise, rather than a satisfactory solution. A special advantage of multi-rotor hammermills is the so-called impact zone between the rotors, in which the material is projected from one rotor against the stream of material projected by the other rotor, and due to this impact zone, double rotor hammermills are less dependent on the hammers having sharp edges and thus require less rewelding. However, in order to avoid excessive grinding action, it has been necessary to arrange a discharge screen for each rotor immediately below the impact zone or area of contiguity between the two rotors. It is also possible to arrange screens below the second rotor at a position spaced circumferentially from the impact zone, but the position of such screens has not yielded the same throughput or as fast an extraction as screens immediately below the impact zone. There is also a possibility that screens remote from the impact zone may choke up or material may slowly build up on them, because such screens are not exposed to the self-cleaning action of the two streams of material projected by the two rotors. Screens at the upper part of the impact zone of the rotors have also been tried, but have proven satisfactory only for material which is quite hard and relatively easy to grind or shatter, since it has not been satisfactory for tough, moist or otherwise difficult material which requires repeated passage through the impact zone until ground finely enough. Suction has also been applied to such overhead screens but has proven to be too close to the feed opening and has upset the balance of the flow around the two rotors.
Additional difficulties with horizontal multi-rotor hammermills have included the inspection and maintenance problem, since access to the hammers and the screens, which are the principal parts requiring periodic inspection, requires the removal of parts surrounding the rotors and requires the removal of numerous cover plates to obtain access thereto. Multi-rotor hammermills in which a first rotor is placed directly above a second rotor have also been tried to improve the ease of access to the rotors. However, these have proved to be vulnerable to vibration, because of insufficient anchorage support of the upper rotor and the increasing amount of vibration caused by both rotors when the hammers begin to show different wear patterns.
All known hammermills, including multi-rotor hammermills, are subject to damage due to heavy tramp metal or other nearly indestructible objects, such as an engine block, getting into the grinding area and particularly into the impact zone of multi-rotor hammermills. Thus, such hammermills have been unable to grind critical material, such as city refuse, finely enough in one stage. As a result, it has been necessary to equip all hammermills subject to this danger and designed to grind to a comparatively small size, with relatively large openings to permit quick discharge of material which cannot be quickly disintegrated.
Among the objects of this invention are to provide a novel multiple rotor hammermill in which the comminuting or grinding action is improved; to provide such a hammermill which is capable of comminuting or grinding wet material, particularly waste paper; to provide such a hammermill which is capable of simultaneously comminuting or grinding numerous different types of material, including hard material, such as glass, ceramic, stone and the like, cellulosic fiber materials, such as newspaper waste, whether wet or dry, relatively soft material, such as food waste or lawn waste, plastic, rubber, leather, aluminum and other nonferrous material, as well as ferrous material when desired; to provide such a hammermill in which access to various parts for inspection or maintenance is greatly facilitated, and particularly for the hammers and screens; to provide such a hammermill in which heavy metal objects passing into the impact zone and likely to cause considerable damage may be automatically ejected; to provide such a hammermill which will combine the advantages of conventional double hammermills without having their shortcomings; to provide such a hammermill in which the impact of the hammers of the second rotor on material being comminuted by the first rotor is enhanced with the material not necessarily passing completely into the area of the second rotor; to provide such a hammermill in which each rotor may be adequately supported and thus resistant to the effects of vibration; and to provide such a hammermill which will be efficient and effective in use and reliable in operation.